Prime Minister Tony Abbott says the multi-billion-dollar program to purchase new jet fighters for the Defence Force does not involve any "new spending" by taxpayers.

The Government has given the go ahead for the purchase of 58 more F-35 Joint Strike Fighters (JSFs) at a cost of $12.4 billion - making it the nation's most expensive Defence asset.

Asked if he was worried taxpayers would question the cost of the program at a time his Government is warning of wide-ranging cuts, Mr Abbott responded: "I want to stress that this is money that has been put aside by government over the past decade or so to ensure that this purchase can responsibly be made."

"This is not new spending today. In the context of a tough budget, this is spending money that we need to spend that has been sensibly put aside in the past to ensure that our nation's defences remain strong."

The extra aircraft will bring Australia's total Joint Strike Fighter force to 72 aircraft, with the first of them to enter service in 2020.

The Prime Minister said the Government was "not going to sacrifice the defence of our nation".

"In the end, government has no higher priority than the defence of the nation, and an effective Defence Force for a country such as Australia requires modern and capable joint strike aircraft such as these. It requires a small but powerful and flexible army and it also requires a strong and effective navy, including a substantial submarine force," he said.

The Government says it will also consider the option of buying another squadron of the next-generation fighter jets to eventually replace the RAAF's F/A-18 Super Hornets.

As part of the announcement, more than $1.6 billion will be spent on new facilities at air bases in Williamtown in New South Wales and Tindal in the Northern Territory.

Former RAAF flight test engineer and the co-founder of military think-tank Air Power Australia, Peter Goon, says the clauses are a necessity given "this has been a program fraught with problems from day one".

"I certainly hope he has clauses in the contract which state if it doesn't shape up to the capability as advertised we're allowed to withdraw," he said.

Mr Goon says the JSF is not fit for the purpose for which it has been purchased.

"[The JSF] has been bought to maintain air superiority for Australia in the region ... however, [it] is not an air superiority fighter and will therefore not provide the ability to maintain and sustain regional air superiority for Australia," he said.

The head of the JSF program, US Air Force Lieutenant General Chris Bogdan, visited Australia earlier this year and declared the reliability and maintainability of the aircraft was not yet "good enough".

In late March the US House Armed Services Committee was told the planes were not affordable to use at the moment.

The committee heard software problems could delay the fighter's production, and foreign buyer delays could see countries like Australia paying millions of dollars more per aircraft.

Liberal backbencher Dennis Jensen, who has repeatedly criticised the JSF, has questioned the timing of the purchase.

"The reality is that we should have, at the very minimum, waited until the aircraft had passed operational testing evaluation in the United States," he told ABC's The World Today program.

"The simple fact is that the earliest that the Air Force is looking at having an initial operational capability with this aircraft is 2020."